The chemical nature of many electrochemical systems employed for the production of electricity is such that gas, usually hydrogen, is generated during storage and sometimes during or following service use. When sealed cell containers are employed, as is the case in certain primary and secondary cells, the build-up of gas-pressure within the sealed container may cause damage to the cell and/or the device in which the cell is employed.
One approach which has been taken in the past to avoid such a potentially dangerous build-up of pressure involves the continuous or periodic release of excess pressure through a resealable valve system. However, while many of these valve-type venting systems will satisfactorily relieve high gas pressures they unfortunately will, in many situations, permit electrolyte leakage. For example, when employed in alkaline cells, release valves which are intended to reseal after activation under pressure are easily fouled by salts formed from caustic electrolytes. These salts are formed when caustic electrolyte, which has crept along the sealing surface of the valve, combines with the carbon dioxide in the air to form a carbonate. The carbonate salt cakes the valve surface and prevents proper resealing of the valve, with the result that more electrolyte will then leak through the partially opened valve. Moreover, most resealable valve systems require the use of additional parts which increase cost and which occupy internal cell volume which could otherwise be filled with active cell material.
A second approach that has been employed in the prior art involves the use of blow out membranes. These membranes have been of two basic types: puncture mechanisms and rupture mechanisms.
Puncture mechanisms employ a spiked member which, when sufficient internal pressure develops, is pushed by such pressure so that the spike punches a hole in a thin membrane located near said spiked member. However, a problem with this mechanism is that the hole may become plugged by the spike and/or by other components from within the cell. Moreover, the design of such puncture mechanisms frequently required that the venting mechanism take up a comparatively large volume of space which could otherwise be employed to contain additional active material within the cell.
Rupture mechanisms employ a seal at least a portion of which is comprised of a thin membrane. This thin membrane ruptures when the interior pressure of the cell becomes too great. In the past, rupture mechanisms have involved the use of either molded or stressed membranes. While rupture mechanisms have the advantage of occupying only a minimal amount of internal cell volume, reliably obtaining uniform venting pressure in either molded or stressed types of rupture mechanisms is difficult, particularly when low pressure venting is desired. For example, in molded membrane constructions (such as that exemplified by U.S. Pat. No. 3,218,197) limitations of the molding process place restrictions on how thin rupture membranes may be molded in addition to creating difficulties in producing a uniform thickness of the membranes such that the blow out pressure will be consistent for a number of cells. Similarly, in stressed rupture mechanisms (such as that shown in German Auslegeschrift No. 1,177,223) the lateral stress exerted along the stressed membrane may vary from cell to cell so as to preclude consistent venting pressures for a number of cells.
It is therefore an object of this invention to provide a galvanic cell having a safety vent mechanism which mechanism occupies only a minimum amount of internal cell volume and which will rupture at a predictable safe predetermined pressure.
It is a further object of this invention to provide a method for the production of a galvanic cell with a nonsealable reliable safety vent mechanism, which mechanism occupies only a minimal amount of internal cell volume.
The foregoing and additional objects of this invention will become apparent from the following description and accompanying drawings and Example.